The present invention relates to a coating composition comprising phosphorus acid-functionalized latex particles, TiO2 particles, a sulfur acid functionalized dispersant, and a carboxylic acid functionalized dispersant.
The improvement of the distribution of TiO2 particles in a paint film is a longstanding goal in the coatings industry. As the concentration of TiO2 in a formulation increases, diminishing opacity returns are observed due to “crowding” of TiO2, wherein clusters of TiO2 particles act as a single, somewhat larger TiO2 particle rather than as independent scattering centers.
Phosphoethyl methacrylate (PEM)-containing binders have proven to be very useful in addressing this problem because they adsorb to the surface of TiO2 particles, resulting in much better spacing and consequently better scattering efficiency. This improved spacing allows the formulator to enhance performance using the same amount of TiO2 or to match opacity at lower TiO2 levels.
In practice, the kinetics of binder adsorption to TiO2 must be tuned properly to obtain these improvements. If adsorption is too slow, no appreciable adsorption will occur during the paint making process; consequently, either the opacity will be indistinguishable from that obtained using a conventional binder, or composite formation will continue to occur after the paint has been fully formulated, resulting in a dramatic rise in viscosity during storage. If, on the other hand, binder adsorption is too rapid, large aggregates of binder and TiO2 (grit) will form, resulting in lower opacity, lower gloss, and poor surface smoothness. When the kinetics are properly optimized, most of the composite particles will consist of latex particles adsorbing to discreet, non-agglomerated TiO2 particles, thereby resulting in higher opacity.
The kinetics of adsorption for a particular TiO2/latex pairing is highly dependent on the surface chemistry of both materials. Consequently, binder suppliers must produce a range of products with different PEM levels for use with the full range of TiO2 grades that paint manufacturers might want to use. Additionally, paint manufacturers may not be able to switch TiO2 grades to take advantage of TiO2 pricing or availability without also switching the binder grade they use.
It would therefore be an advantage in the art to provide a way for formulators to readily optimize the kinetics of adsorption for a wide array of adsorbing binders and TiO2 grades.